Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
A digital micromirror device (DMD.TM.), sometimes referred to as deformable micromirror device, is a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics were used to illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
At each instant, the light incident a DMD element is either reflected toward the image pixel, or is reflected away from the image pixel. Thus, the pixel associated with the DMD cell, that is the pixel created with light reflected by the DMD mirror, is either maximally illuminated ("on") or minimally illuminated ("off"). Intermediate intensities are created by pulse width modulating the mirror element over a frame time so that the duty cycle of the mirror element is proportional to the desired optical density of the pixel, as indicated by an image data word for the pixel.
Because the electrostatic operation of the DMD latches the mirror in place, binary data for one bit period is loaded into a memory cell addressing each mirror element during the prior bit display period. Unfortunately, the time required to load the entire array, or portion of the array, called the load period, can be longer than the display period of one or more of the LSBs. When this occurs, new data cannot be loaded during the previous display period, and a blanking period is required. A blanking period occurs when off data is loaded into the entire array, or a reset group of the array, and the mirrors in the reset group are turned off while data is loaded into the reset group. The data for the blanking period can be loaded into the array faster than actual image data because the memory cells for many rows of mirror array are loaded simultaneously.
Loading many rows of the memory cell array at the same time can require a large drive current for the bitline driver. Since the physical area in which the driver is located is very limited, it is difficult to provide a large current driver. Instead, past DMDs have relied on an extended write time to allow the bitline driver to overcome the capacitance of the many memory cells that it is attempting to drive. What is needed is a better system and method for driving the memory array bitline that will allow many rows of the mirror memory array simultaneously to be driven without extended write times.
Loading many rows of a memory cell array can cause a large peak current depending on the state of the memory cells. This peak current, coupled with the RC time constant of the bitline, can overpower the bitline driver and cause wrong data to be loaded into the memory cells. The five-transistor SRAM cell is especially susceptible to wrong data when loaded in this fashion. The bitline can be thought of as a distributed RC load. Therefore, memory cells located furthest from the bitline driver are most likely to be loaded with wrong data when multiple rows of memory cells are being loaded.